One kind of defect that can occur during the manufacture of cables is an opening in a plastic jacket which encloses a core that comprises a plurality of individually insulated conductors. Unwanted jacket openings may be caused in several ways during the manufacture of a cable, such as, for example, by line and drive system disturbances. An incomplete breakup of plastic pellets, which are supplied to a jacketing extruder, or a loose binder wrapped about the core can cause a protuberance such that a plastic melt comprising an extrudate tears about the protuberance as it is pulled down into engagement with the core.
Such openings are more likely to occur in thin-wall jackets, i.e. on the order of 20 to 40 mils thick, which are customary for inside wiring and switchboard cables, than in those cables used in other environments where jacket thicknesses are generally on the order of 45 to 100 mils. This problem has become more critical in recent years because of plastic material shortages. With petroleum-based plastics not infrequently in short supply, there has been a trend toward the use of thinner jackets of perhaps different plastics which offer the same amount of mechanical and electrical protection notwithstanding the thinner wall.
Until recently, there had been no industry wide effort in telecommunications cable manufacture to detect jacket openings along a manufacturing line on which a cable core is jacketed. This may be due, at least in part, to the heretofore uninterrupted supply of plastic and hence the absence of a need to minimize jacket thicknesses.
Commercially available equipment which generally is designed to detect coaxial capacitance unbalance caused by jacket openings does not consistently and reliably differentiate between jacket openings and noise. Coaxial capacitance as measured from a probe to the cable core varies because the jacket is not always concentrically disposed about the core with the jacket thickness varying within acceptable limits. Inasmuch as capacitance is an area-sensitive electrical characteristic, a further drawback to the use of capacitance unbalance to detect jacket openings is that an opening of significant size must occur to cause a measurement reading to vary beyond its normal range.
The prior art includes U.S. Pat. No. 3,047,799 which discloses interference-free spaced dual probes for detecting deleterious occlusions within insulated conductors. The methods used in this patent require (1) that the conductor under test be grounded, (2) that a high voltage be applied as the conductor is passed through the dual probe to cause corona at the insulation occlusion, (3) that distilled water be used as a couplant in order to obtain a high resistance between an exciting ring and the insulated conductor under test, and (4) that a center conductor be excited and that signals radiated from the occlusions be detected.
Also included in the prior art is a bare wire detecting device which includes two conductive zones that are insulated and spaced from each other by a predetermined distance. A relatively high D.C. voltage is impressed across the two zones to cause bare portions of an insulated conductor which are equal to or longer than the distance between the zones to complete a circuit to indicate the presence of unacceptable lengths of bare wire. See U.S. Pat. No. 3,277,365. This device is inapplicable to a test for jacket openings since there is no bare wire; moreover, this arrangement, which relies on high voltage for its test, is not desirable for use in a factory environment.
It is also old to use light to detect the presence of pinholes during the manufacture of a metallic strip such as tinplate which may be moved at speeds up to approximately 1524 meters per minute. The use of a light source and a conventional detector tube on opposite sides of a jacketed cable core would not be adaptable to the detection of jacket openings for obvious reasons.
Nowhere in the prior art does there appear to be a simple, on-line apparatus which may be used to detect jacket openings without the necessity of special couplants such as distilled water in cooling troughs, without the necessity of relying on an operator for externally grounding the cable, and without the need to externally excite the cable under test.